1. Field of the Invention
This invention relates to liquid, phosphate-free single phase degreasing compositions which may be used for cleaning aluminum surfaces.
2. Statement of Related Art
The application of inorganic or organic coatings to metal surfaces, which has recently acquired increasing significance in efforts to improve corrosion prevention or to obtain decorative effects, requires careful cleaning of the metal surfaces beforehand in preparation for the application of the coatings. Metal surfaces are normally treated with highly alkaline cleaning solutions with more or less complete removal of soil, particularly grease.
Cleaning with strongly alkaline cleaning solutions has always been problematical in the case of aluminum surfaces or metal surfaces containing aluminum in addition to other metals, because aluminum is known to be extremely sensitive to strongly alkaline aqueous solutions. At extremely high pH values, undesirably large quantities of aluminum are dissolved from the metal surfaces in the form of alkaline aluminate complex salts. Accordingly, where aluminum surfaces are cleaned with alkaline cleaning preparations, it has always been necessary to strike a compromise between the degreasing performance of the cleaning composition on the one hand and the undesirable erosion of metal from the aluminum surfaces on the other hand. Aqueous systems in which builders suitable as buffers are used are normally employed for cleaning aluminum surfaces. In the present context and in the context of the following description and in the claims, "builders" are understood to be compounds which are active as buffers and which enhance the cleaning effect of surfactants. Builders which have been successfully used in the prior art for the cleaning of aluminum surfaces include, in particular, borate salts such as borax, which are used in combination with alkali metal orthophosphates or alkali metal salts of condensed phosphates and which developed an adequate degreasing effect without excessive quantities of metal being eroded from the aluminum surface.
The principal disadvantage of known formulations of this type is that the corresponding borate salts are poorly soluble in water. The effect of this is that problems are repeatedly encountered in the preparation of the known solutions because multiphase systems are formed during dosing of the (generally powder) compositions in process water. In addition, the condensed phosphates normally used as sequestering (complexing) agents are gradually hydrolyzed in aqueous solution, losing their complexing properties in the process. In addition, the phosphates formed in the known solution and in the wastewater are also ecologically undesirable and have to be gradually replaced by other compounds to be able to prevent the eutrophication of surface waters.
The cleaning preparations previously proposed in the prior art are unsuitable for solving the problems involved in the cleaning of aluminum surfaces for a number of reasons. For example, U.S. Pat. No. 4,521,332 describes aqueous compositions for the cleaning of metal surfaces which are strongly alkaline and which contain large quantities of sodium hydroxide as well as an alkali metal carbonate dispersed in polyacrylic acid. Cleaning dispersions such as these are unsuitable for the degreasing of aluminum surfaces simple because of their high alkalinity.
U.S. Pat. No. 4,528,039 describes compositions intended for the degreasing of aluminum surfaces which, in addition to sodium carbonate, contain sodium silicate as a builder. Surface active agents and other additives known from the prior art are also present in the compositions. However, compositions such as these cannot be used in dissolved form for the degreasing of aluminum surfaces because, as powders, they have to be introduced into the aqueous phase in a certain quantity before application. Complete dissolution or rather homogeneous dispersion of the compositions in the aqueous phase is not guaranteed, at least on an industrial scale. In addition, compositions of the type in question are attended by the disadvantage that automatic dosing of powders is not readily possible. Because of this, liquid formulations are preferred for industrial application.
According to H. -G. Germscheid "Untersuchungsmethoden bei der Entfettung" in "Gavanotechnik" 67, 215 (1976), surfactants play an important part in the degreasing and cleaning of metal surfaces insofar as they displace the film of grease adhering to the metal surfaces and thus enable the metal surfaces to be more or less completely degreased. Degreasing in the adsence of surfactants is not regarded as possible in this publication. In particular, it is shown by the results of experiments that the effect of the surfactants can be enhanced by the effect of other constituents, particularly builders, in cleaning preparations. The test results described in the above article also provide quantitative proof of the synergistic effect of builders and surfactants.